The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An internal combustion engine (ICE) is powered by combustion energy generated in cylinders of the ICE. The cylinders have respective intake and exhaust valves. An air/fuel mixture may be received through the intake valves, and exhaust gas is removed from the cylinders through exhaust valves. The intake and exhaust valves may be actuated by cams on a camshaft. The camshaft may be driven by a crankshaft via one or more timing belts, gears and/or chains. The cams open the intake and exhaust valves at respective angular displacements of the camshaft during intake and exhaust strokes. The opening of a valve may include the lifting of a valve away from a cylinder, referred to as valve lift. The amount of time that a valve is opened is referred to as valve lift duration.
Extent of valve lift and valve lift duration may be based on a cam profile. The cam profile, characterized by contour shape and angular position relative to a camshaft, may be designed for a particular engine operating at a particular engine speed. Once the cam profile is determined and the engine is assembled, the cam profile may be used for all engine operating speeds.
A fixed cam profile may provide limited performance for speeds other than the particular engine speed for which the fixed cam profile was designed. Since a cam opens intakes valves based on angular displacement of a camshaft, duration of intake valve opening can be reduced as speed of the camshaft increases. The reduced duration in intake valve opening can negatively affect engine performance. For example, an engine may need an increased amount of air when operating at an increased engine speed. For the stated reasons, a fixed cam profile that is designed for a engine speed range may not provide sufficient valve opening or lift to allow a requested amount of air to be fully received when an engine speed is greater than the engine speed range.
Variable valve lift (VVL) control systems allow multiple cam profiles to be selected to actuate intake and exhaust valves. The VVL control system may provide a larger amount of valve lift at higher engine speeds than at lower engine speeds by applying different cam profiles. Variable valve lift control can improve engine performance including increased efficiency and reduced emissions.